The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The demand for eco-friendly vehicles is increasing due to the constant demand for fuel efficiency improvement for vehicles and the strengthening of exhaust gas regulations in many countries. As a practical alternative to engine-driven vehicles, a hybrid electric vehicle/plug-in hybrid electric vehicle (HEV/PHEV) is provided.
Such a hybrid vehicle can provide optimal output and torque depending on how well the engine and motor are operated in harmony in the course of driving with the two power sources. Particularly, in a hybrid vehicle adopting a parallel type hybrid system in which an electric motor and an engine clutch (EC) are mounted between the engine and the transmission, the output of the engine and the motor can be simultaneously transmitted to a drive shaft.
Generally, in a hybrid vehicle, electric energy is used during initial acceleration (i.e., EV mode). However, since electric energy alone has a limitation in meeting the required power from drivers, use of the engine as the main power source is eventually required (i.e., HEV mode). In such a case, in the hybrid vehicle, when the difference between the number of revolutions of the motor and the number of revolutions of the engine is within a predetermined range, the engine clutch is engaged so that the motor and the engine rotate together. Such a hybrid vehicle structure will be described with reference to FIG. 1.
FIG. 1 shows an example of a power train structure of a general hybrid vehicle.
FIG. 1 illustrates the power train of the hybrid vehicle adopting a parallel-type hybrid system in which an electric motor (or a drive motor) 40 and an engine clutch 30 are installed between an internal combustion engine (ICE) 10 and a transmission 50.
Typically, when a driver presses an accelerator (e.g., accelerating pedals) after starting the vehicle, the motor 40 is first driven using the electric power of a battery in the state in which the engine clutch 30 is opened, and wheels move by power transferred to a final drive (FD) 60 via the transmission 50 from the motor (i.e. EV mode). When a larger driving power is required due to the gradual acceleration of the vehicle, the engine 10 may be driven by operating an auxiliary motor (or a starter/generator motor) 20.
Thus, when the RPMs of the engine 10 and the motor 40 are equal to each other, the engine clutch is in an engaged state so that the vehicle is driven by both the engine 10 and the motor 40 (i.e. transition from EV mode to HEV mode). When a predetermined engine off condition, such as the deceleration of the vehicle, is satisfied, the engine clutch 30 is opened and the engine 10 is stopped (i.e. transition from HEV mode to EV mode). In this case, the battery is charged through the motor using the driving force of the wheels in the vehicle, which is referred to as braking energy regeneration or regenerative braking. Accordingly, the starter/generator motor 20 serves as a starter motor when the engine is started, and serves as a generator when the rotational energy of the engine is recovered after starting or during starting off. Therefore, the starter/generator motor 20 may be referred to as a Hybrid Start Generator (HSG).
In general, the transmission 50 uses a step-variable transmission or a multi-plate clutch such as a dual clutch transmission (DCT), and is shifted to 2nd step in accordance with the speed and torque after starting in the 1st step in EV mode. At this time, in order to smoothly change gears and protect the clutch in the upper shifting, the vehicle is controlled to reduce the transmission input shaft speed, such as reducing the torque of the drive source. Such control may be referred to as “intervention control”.
For example, reverse torque may be applied by the electric motor 40 as the driving source torque reducing means. In this case, the electric power may be generated in the electric motor 40. This will be described with reference to FIG. 2.
FIG. 2 shows an example of an intervention process for an upper shift in a general hybrid vehicle.
Referring to FIG. 2, three graphs are shown, and the vertical axis, from top to bottom, shows the speed of the intervention, the torque of the electric motor, and the speed of the transmission input shaft, respectively.
The shifting process can be classified into a torque phase and an inertia phase. The torque phase may mean a phase in which the speed of the input shaft rises by a positive torque generated in an electric motor. The inertia phase may mean a phase at which the torque of the motor is reduced and the speed of the input shaft is reduced. Further, application of reverse (−) torque to an electric motor may mean power regeneration. Thus, the electric power generated by the electric motor can be used for charging the battery.
On the other hand, when switching from EV mode to HEV mode is determined at the acceleration in the hybrid vehicle, the engine is started. As described above, engine starting involves cranking using the power of the HSG. This will be described with reference to FIG. 3.
FIG. 3 shows an example of an engine starting process in a general hybrid vehicle.
In FIG. 3, the vertical axis of the upper graph represents the torque of the HSG, and the vertical axis of the lower graph represents the engine speed. Referring to FIG. 3, the engine is cranked by the torque generated in the HSG, and then the engine is started.
The condition for switching from EV to HEV mode is determined by various factors such as the battery's state of charge (SOC), auxiliary load, torque demand, etc. However, in the normal acceleration situation, the upper shift from the first stage to the second stage and the engine cranking occur at a similar point in time. However, in the general hybrid vehicle, as shown by the arrow in FIG. 1, the electric energy E recovered in the shifting process first charges the battery 70, and the electric power stored in the battery 70 is again supplied to the HSG 20. Therefore, only the value ηinE obtained by multiplying the input efficiency factor ηin (ηin<1) of the shift recovery energy E is stored in the battery 70. Further, when the electric power ηin E stored in the battery 70 is outputted from the battery 70, it is multiplied again by the output efficiency factor ηout (ηout<1). As a result, there is a problem that the energy E recovered at the time of shifting is transmitted to the HSG only by E*ηin*ηout due to the path loss.